1. Field of the Invention
The present invention relates to a method of forming an ohmic electrode and a semiconductor light emitting element, and more particularly, to a method of forming an ohmic electrode formed on a semiconductor layer to allow a current applied thereto, and a semiconductor light emitting element using the ohmic electrode.
2. Discussion of the Background
Semiconductor light emitting elements have advantages of small size, lightweight, low power consumption and long life span, and do not require preheating time and complicated driving circuits. Further, since the semiconductor light emitting elements are strong in shock and vibration and can be packaged into various shapes, the semiconductor light emitting elements is expected to substitute for backlights of large-sized liquid crystal displays, general illumination and light sources for vehicle headlights within next few years.
Particularly, since a nitride-based semiconductor light emitting element has excellent characteristics of electron affinity, electron mobility, electron saturation velocity and electric field breakdown voltage, high efficiency and high power can be implemented. Since the nitride-based semiconductor light emitting element does not contain harmful substance such as As or Hg, it has been noticed as an environment-friendly element.
In general, a nitride semiconductor light emitting element is fabricated by sequentially laminating a nitride-based n-type layer, a nitride-based active layer and a nitride-based p-type layer on a sapphire substrate, and then arranging two electrodes horizontally so as to apply power to the n-type and p-type layers. Such a light emitting element with a horizontal structure is relatively simple in a fabrication process and thus is inexpensive, while it is difficult to implement high power. That is, since light generated from the active layer is absorbed in the two electrodes and thus is not emitted to the outside, the high power cannot be implemented. There is also a problem in that since a sapphire substrate has a low thermal conductivity and thus heat generated in an operation process is not effectively emitted, the thermal stability lowers.
In order to solve such a problem, a light emitting element with a vertical structure and a flip-chip type light emitting element have been suggested. In this case, a reflective layer is formed on any one electrode to allow light generated from an active layer to be easily emitted to the outside, whereby the light availability can be enhanced. Further, a metal substrate with excellent thermal conductivity is used in place of a sapphire substrate, whereby the thermal stability can be enhanced.
However, nitride semiconductor light emitting elements that have been developed up to now are still unsatisfactory in view of high power, light emitting efficiency and price, and their performance should be more improved. Particularly, in order for the light emitting elements to substitute for conventional mercury lamps and fluorescent lamps, a problem of high power and thermal stability in accordance therewith should be solved.
Meanwhile, in order to obtain higher power, the development of an electrode with a high light reflectivity should be preceded. Since a metal such as Al or Ag is excellent in a visible region in view of light reflectivity, an excellent characteristic of light output can be obtained by using such a metal as an electrode. However, since Al has large contact resistance with a nitride-based semiconductor layer, it is difficult to apply large current. Since Ag has lower contact resistance but has poor interlayer adhesive strength and low thermal stability, there is a problem in that agglomeration, interface voids and the like are formed during heat treatment.
Due to such problems, Au- or Pt-based electrodes used for light emitting elements with a horizontal structure have still been used for light emitting elements with a vertical structure. Therefore, there is a limit in that light output is secured to the extent that the light emitting elements substitute for conventional white light sources. Further, since the Au or Pt, which is a conventional electrode material, is expensive, there is a problem in that the fabrication cost is increased.